In the quest to obtain inexpensive and reliable energy, some nuclear reactors have been designed with the goal of being passively operated. In these passive systems, the laws of physics may be employed to ensure that safe operation of the nuclear reactor is maintained during normal operation or even in an emergency condition without operator intervention or supervision, at least for some predefined period of time. One goal of the passive operating systems is to minimize the number of motors, pumps or other electrical or mechanical devices which have traditionally been relied upon to operate the nuclear reactor.
A Multi-Application Small Light Water Reactor project conducted with the assistance of the Idaho National Engineering and Environmental Laboratory, NEXANT and the Nuclear Engineering Department of Oregon State University sought to develop a safe and economical natural light water reactor. FIG. 1 illustrates a nuclear reactor design 20 that resulted from this project.
The nuclear reactor design 20 includes a reactor core 6 surrounded by a reactor vessel 2. Water 10 in the reactor vessel 2 surrounds the reactor core 6. The reactor core 6 is further located in a shroud 22 which surround the reactor core 6 about its sides. When the water 10 is heated by the reactor core 6 as a result of fission events, the water 10 is directed from the shroud 22 up into an annulus 23 located above the reactor core 6, and out of a riser 24. This results in further water 10 being drawn into the shroud 22 to be heated in turn by the reactor core 6 which draws yet more water 10 into the shroud 22. The water 10 that emerges from the riser 24 is cooled down and directed towards the outside of the reactor vessel 2 and then returns to the bottom of the reactor vessel 2 through natural circulation. Pressurized steam 11 is produced in the reactor vessel 2 as the water 10 is heated.
A heat exchanger 35 circulates feedwater and steam in a secondary cooling system 30 in order to generate electricity with a turbine 32 and generator 34. The feedwater passes through the heat exchanger 35 and becomes super heated steam. The secondary cooling system 30 includes a condenser 36 and feedwater pump 38. The steam and feedwater in the secondary cooling system 30 are isolated from the water 10 in the reactor vessel 2, such that they are not allowed to mix or come into direct contact with each other.
The reactor vessel 2 is surrounded by a containment vessel 4. The containment vessel 4 is placed in a pool of water 16. The pool of water 16 and the containment vessel 4 are below ground 28 in a reactor bay 26. The containment vessel 4 does not allow any water or steam from the reactor vessel 2 to escape into the pool of water 16 or the surrounding environment. In an emergency situation, steam 11 is vented from the reactor vessel 2 through a steam valve 8 into an upper half 14 of the containment vessel 4, and water 10 flashes as it is released through a submerged blowdown valve 18 which is located in a suppression pool 12. The suppression pool 12 includes sub-cooled water. Over pressurization of the reactor vessel 2 is therefore reduced by releasing both steam 11 through the steam valve 8 and water 10 through the blowdown valve 18 into the containment vessel 4. The rates of release of the steam 11 and water 10 into the containment vessel 4 vary according to the pressure within the reactor vessel 2. Decay heat is removed from the reactor core 6 through a combination of condensation of the steam 11 and energy transfer of the water 10 to the suppression pool water 12.
The water in the suppression pool 12 provides pressure suppression and liquid makeup capabilities in the event of a loss of coolant or pipe rupture in the containment vessel 4. However this also means that electrical and mechanical components in the containment vessel 4 are constantly subject to a corrosive environment, which introduces reliability issues. Insulation that surrounds the reactor vessel 2 loses some of its insulating properties when located in a wet or humid environment, and may need to be replaced at regular intervals. Expensive and exotic materials may be used for the reactor vessel insulation. In addition, maintenance, monitoring and inspection of the electrical and mechanical components must be performed to ensure their continued reliability of operation.
The present invention addresses these and other problems.